Implantable defibrillation systems, as one instance of implantable medical devices, are known in the art which deliver a high-voltage defibrillation pulse to the heart when the onset of fibrillation is detected and/or in the event of a detected complete loss of cardiac output. Such known devices are also capable, if the heart exhibits an arrhythmia such as atrial fibrillation, atrial flutter or tachycardia, or ventricular tachycardia, of cardioverting the heart by delivering a low-voltage pulse in an attempt to regain synchronous operation of the heart, instead of delivering the high voltage defibrillation pulse. In known devices of this type, considerable effort has been devoted to the development of detection circuitry to accurately identify heart arrhythmias which require defibrillation or cardioversion (i.e., insuring true-positives) and for preventing a "no output" situation when an output is actually needed (i.e., preventing true-negatives). Steps have also been taken to prevent the delivery of a defibrillation or cardioversion pulse when none is needed (i.e., preventing false-positives). See, for example, commonly assigned U.S. Pat. No. 5,190,034, to Sholder.
It is important to provide a safety mechanism for preventing implantable systems of this type from releasing a treatment pulse when-the patient feels no need for such a pulse. A false-positive output could result in severe discomfort to the patient, and may trigger ventricular tachycardia, ventricular fibrillation and ultimately death if the system cannot react quickly enough to provide proper treatment to revive the patient.
In U.S. Pat. No. 3,841,305, to Hallgren, a system is disclosed for the external stimulation of a nerve and includes a coil of wire with a flux-concentrating core in the lumen of the coil. The core preferably has a T-shape with the base of the T extending through the lumen of the coil providing the area of stimulation. The coil is pulsed by a discharging capacitor and circuitry is disclosed for charging the capacitor and generating discharge pulses of alternate polarity.
It is known from U.S. Pat. No. 4,086,916, to Freeman et al., to contain a cardiac monitoring system in a wristwatch worn by a patient, the system including circuitry for detecting an erratic heartbeat, a missing pulse or other irregularities and providing an alarm indication, audio and visual, when such an event is detected.
It is also known from U.S. Pat. No. 4,088,139, to Auerbach, to provide, in an implantable cardiac pacing system, means for generating a marking pulse in the defibrillator monitoring system if an event such as loss of capture occurs. The patient is not immediately informed of the occurrence of such an event, however, the system is provided with telemetry means so that when the recorded data is subsequently read out and examined by a physician, the data will include the marker indicating that loss of capture has occurred. The physician can then take such corrective steps as may be necessary.
An implantable pacing system is disclosed in U.S. Pat. No. 4,102,346, to Fulker, which includes an alarm device as part of the implanted unit which generates an alarm signal to inform the defibrillator user when the battery source of power of the defibrillator is nearing end-of-life or is malfunctioning.
An implantable tissue stimulating device is disclosed in U.S. Pat. No. 4,345,603, to Schulman, which activates an alarm which informs the patient in whom the system is implanted that the battery is in need of replacement. After the user has been so informed, the user applies a magnet externally in the vicinity of the implanted unit to deactivate the monitoring system and thereby cease the continued operation of the alarm.
A pacemaker for controlling tachycardia is disclosed in U.S. Pat. No. 4,407,289, to Nappholz et al., also disclosing means for informing a pacemaker user of the remaining battery life. The user places a magnet externally in the vicinity of the implanted unit, which thereby causes the implanted unit to generate two pulses, which can be seen on the patient's ECG waveform. The time separation between the two pulses indicates the remaining battery potential. Application of the magnet, after the pulses have been generated, temporarily disables the device.
U.S. Pat. No. 4,892,100, to Schaldach, discloses a demand pacemaker with physiological control intended to simplify adaptation to the individual patient and thus make it possible that, in addition to initial adaptation to the individual stimulation requirements, the operating behavior of the pacemaker can continuously be brought up to date to correspond to changing demands for stimulation.
In U.S. Pat. No. 5,190,034, noted earlier, an implantable arrhythmia treatment system is disclosed which includes reliable protection against the release of unneeded treatment pulses, that is, which provides protection against a false-positive output. The disclosed system utilizes an alarm generator, which may be disposed in the implanted unit, or in an external unit. The alarm may be of any type which does not require constant, active monitoring by the user, such as a sensory alarm, for example, an audio alarm generator or a tactile alarm generator or "tickler".
Other examples of implantable arrhythmia devices which include an alarm generator, either audio, tactile, or visual, are found in U.S. Pat. Nos. 4,295,474, to Fischell; 4,210,149, to Heilman et al.; and 3,783,877, to Bowers.
According to the current state of the art, error conditions are typically announced within an implantable cardioverter defibrillator (ICD) using a piezo annunciator or beeper. The current Eagle Model 2800 in development by Pacesetter, Inc., a St. Jude Medical Company, Sylmar, Calif., utilizes a piezo actuator to flex the titanium can at audio frequencies. However, the efficacy of audio emissions from devices implanted abdominally can be questioned. The attenuation of the audio transmitted through tissue is dramatic. Aged patients commonly have hearing loss that further decreases their sensitivity to implanted audio generators.
It was in light of the foregoing that the present invention was conceived and has now been reduced to practice.